Changes in secondary metabolites of green tea during fermentation by Aspergillus oryzae and its effect on antioxidant potential

Metabolomic differences between green tea (GT) and Aspergillus oryzae-fermented green tea (FGT) were investigated using Liquid Chromatography–Electrospray Ionization-Ion Trap–Tandem Mass Spectrometry (LC-ESI-IT-MS/MS). To identify the metabolomic differences, principal component analysis (PCA) and orthogonal projection to latent structures discriminate analysis (OPLS-DA) of GT and FGT were performed. A total of 17 metabolites differed between GT and FGT. The major flavonoid compounds of GT, epigallocatechin gallate (EGCG) and epicatechin gallate (ECG), decreased during fermentation, while the levels of phenolic compounds, such as gallic acid, 3-p-coumaroylquinic acid, and other flavonoid metabolites like gallocatechin, epicatechin, and epigallocatechin, increased significantly during fermentation. Notably, caffeine degradation was also observed during fermentation of GT by A. oryzae. Although the total flavonoid content of FGT decreased, the antioxidant activity of FGT increased significantly due to increased levels of other identified and unidentified metabolites. These results show that fermentation-dependent metabolomic changes have the potential to increase the bioactivity of GT.